1. Field of the Invention
The invention relates to the field of solid-state high-voltage pulse power generators.
2. Description of the Related Art
High voltage pulse generators with fast rise time are typically used to drive gas discharge loads such as lasers, devices for pollution control applications and for plasma chemistry. In the past, such pulse generators used thyratrons to generate the desired pulses. However, thyratrons are relatively unreliable and they have high complexity resulting in a high price. Solid state devices such as thyristors, MOSFET transistors and Insulated Gate Bipolar Transistors (IGBTs), although much lighter and more reliable than thyratrons, handle less power than a thyratron. In order to replace one thyratron, one needs hundreds or thousands of solid state devices.
The power that each solid state transistor can produce is restricted by its saturation current (˜70 A) and by its breakdown voltage (˜600V), giving a maximum pulsed power Pav=Isat×Ubd/2=21 KW. For most plasma applications, the desired pulse length is about 100 nanoseconds. This means that the pulse energy the transistor can produce is typically not higher than 2.1 mJ. If the repetition rate is increased to 50-100 kHz, then the power will have a reasonable value (100 w-200 w). The main advantage of short pulses (<100 ns) in plasma devices is the high electrical strength of the gap that permits achievement of twice or triple the mean temperature of electrons. If the repetition rate is increased to more than 1000 Hz, the residual ionization will reduce the electrical strength of the plasma gap and eliminate the advantage of short pulses. This problem is typically solved by increasing the pulse length of the solid-state pulse generator and using one more output stages to compress the power. The second stage compressor, if desired, can be a spark gap, blumline, etc. To simplify the design of the compressor, the pulse length of the charger is typically held to a few microseconds or even as short as 1-2 microseconds.
Even when using an output compressor, in order to replace one thyratron one needs hundreds or thousands of solid state devices. The cost of all these devices is comparable to the cost of one thyratron, but the reliability of solid state devices is higher. Unfortunately, summing the output of a large number of transistors via a transformer is problematic because the transformer can cause dangerous overvoltage in the transistors.